1. Field of the Invention
The present invention relates to optical coupling into a planar waveguide and, in particular, to a mode size converter for coupling light into and out of a planar waveguide and to efficiently transform guided light within continuous waveguide structures.
2. Discussion of Related Art
The increasing prevalence of fiber optic communications systems has created an unprecedented demand for devices for processing optical signals. Planar devices such as optical waveguides, couplers, splitters, and amplifiers, fabricated on planar substrates, like those commonly used for integrated circuits, and configured to receive and process signals from optical fibers are highly desirable. Such devices hold promise for integrated optical and electronic signal processing on a single semiconductor-like substance.
The basic design of planar optical waveguides and amplifiers is well known, as described, for example, in U.S. Pat. Nos. 5,119,460 and 5,563,979 to Bruce et al., U.S. Pat. No. 5,613,995 to Bhandarkar et al., U.S. Pat. No. 5,900,057 to Buchal et al., and U.S. Pat. No. 5,107,538 to Benton et al., to cite only a few. These devices, very generally, include a core region, typically bar shaped, of a certain refractive index surrounded by a cladding region of a lower refractive index. In the case of an optical amplifier, the core region includes a certain concentration of a dopant, typically a rare earth ion such as an erbium or praseodymium ion which, when pumped by a laser, fluoresces, for example, in the 1550 nm and 1300 nm wavelength ranges used for optical communication, to amplify the optical signal passing through the core.
Many designs have been disclosed for the purpose of mode size or spot size conversion in planar optical wave guide devices. In general, a change in the core of the wave guide, either to its index or to its cross sectional dimensions, over a suitably long distance is utilized to effect a change in the mode size or mode order of the guided light wave. The requirement for gradual change in the properties that govern the mode size is well understood with regard to efficient or ‘adiabatic’ transformation, which results in loss-less conversion of the guided light to a mode having transformed properties such as size, order, shape or propagation constant.
In practice however, it is difficult to change the dimensions of a planar wave guide without loss due to roughness or to non-uniform changes in the wave guide. In particular, sufficiently slowly varying dimensions with a sufficiently smooth surface, so that the light is efficiently converted in its modal properties, without scattering or loss due to roughness or to non-uniform changes in the dimensions or index of the wave guide is very hard to achieve, particularly for high contrast or refractory wave guide materials.
In-plane (i.e., in the plane of the substrate) as well as out-of-plane (i.e., perpendicular to the plane of the substrate) tapers can be achieved by lithographic and etching means. However, it is difficult to achieve sufficiently uniform results for a portion of a film or wave guide tapered over a few mm to a few cm regions across, for instance, a production silicon wafer having dimensions of 100 to 300 mm.
Therefore, there is a need for tapered waveguides with low surface roughness in order to provide mode size conversion with low coupling loss.